Operating reserve
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In power systems, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand, in case a generator is lost or there is another disruption to the supply. Most power systems are designed so that, under normal condtions, the operating reserve is always at least the capacity of the largest generator plus a fraction of the peak load.[1]
The operating reserve can be divided into two types of reserve power: the spinning reserve and the non-spinning or supplemental reserve.
The spinning reserve is the extra generating capacity that is available by increasing the power output of existing generators that are already connected to the power system. For most generators, this increase in power output is achieved by increasing the torque applied to turbine's rotor.[2]
The non-spinning or supplemental reserve is the extra generating capacity that is not presently connected to the system but can be brought online after a short delay. In isolated power systems, this essentially equates to the power available from fast-start generators.[3] However in interconnected power systems, this may include the power available on short notice by importing power from other systems or retracting power that is presently being exported to other systems.[4]
Generators that intend to provide either spinning and non-spinning reserve should be able to reach full capacity within ten or so minutes. Most power system guidelines require a significant fraction of their operational reserve to come from spinning reserve.[5] This is because the spinning reserve is slightly more reliable (it doesn't suffer from start-up issues) and can respond immediately where as with non-spinning reserve generators there is a delay as the generator starts-up offline. Non-spinning reserve is generally cheaper than spinning reserve.[6]
In addition, there are two other forms of reserve power that are often discussed in combination with the operational reserve: the frequency-response reserve and the replacement reserve.
The frequency-response reserve is provided as an automatic reaction to a loss in supply. It occurs because immediately following a loss of supply, the generators slow due to the increased load. To prevent this slowing, many generators have a governor. These governors kick-in almost immediately providing a small boost to both the output frequency and power of each generator. However because the frequency-response reserve is often small and not at the discretion of the system operator it is not considered part of the operating reserve.[7]
The replacement reserve is reserve power provided by generators that require a long start-up time (typically thirty to sixty minutes). It is used to relieve the generators providing the spinning or non-spinning reserve and thus restore the operating reserve (confusingly the replacement reserve is sometimes known as the 30 or 60-minute operating reserve).[8]
The time periods over which all four kinds of reserve power operate is illustrated in the diagram that accompanies this article.[9]
References
- ^ Jianxue Wang; Xifan Wang and Yang Wu, Operating Reserve Model in the Power Market, IEEE Transactions on Power Systems, Vol. 20, No. 1, February 2005.
- ^ Spinning Reserve and Non-Spinning Reserve, California ISO, January 2006.
- ^ Ibid.
- ^ WSCC Operating Reserve White Paper, Western Systems Coordinating Council, July 1998.
- ^ Spinning Reserve and Non-Spinning Reserve, California ISO, January 2006.
- ^ The Value of Reliability in Power Systems, MIT Energy Laboratory, June 1999.
- ^ Eric Hirst, Price-Responsive Demand as Reliability Resources, April 2002.
- ^ Ibid.
- ^ B. J. Kirby, Spinning Reserve From Responsive Loads, Oak Ridge National Laboratory, March 2003.